Although I was never successful in getting my own wife to pose for pre baby shots, I was privileged to be able to take a series of these shots recently.
The most obvious thing to remember with this type of shooting is to put mom in a favorable light. Don't exaggerate her size or distort her size. Do find ways to emphasize love and joy.
Originally I was hired just to do Mom, but it ended up being a family shoot. Which in my opinion is a good thing. A new life in the family effects everybody, so why not include everybody?
If you can get your foot in the door by shooting baby before he or she gets here, who do you think the family will call when baby is here? Shooting a dozen or more pregnant women will not only get you better known, it may also bring you clients for many years to come.